This invention relates to an exposure apparatus and, more particularly, to an exposure apparatus suitably usable in a lithographic process of device manufacturing processes for a semiconductor device such as an IC or LSI, a liquid crystal device, an image pickup device such as a CCD or a magnetic head, for example.
With the need of miniaturization of an IC pattern, strict precision is required to exposure non-uniformness in a lithographic process. For a dynamic RAM of 256 MB, as an example, a line width processing precision of 0.25 micron is required and, in this case, a tolerable exposure non-uniformness is estimated as about 1%.
Also, with respect to the wavelength of exposure light, for enhancement of resolution, those light sources which provide light of a shorter wavelength than i-line of conventional Hg lamps have been recently used. Excimer lasers are a typical example of them. However, excimer lasers are interrupted light emission type lasers, and they produce light of pulses. At an upper limit level, for light emission spacings of about 2.5 msec., the duration of light emission is about several tens of nsec. Further, there is a problem that the emitted light intensity of each pulse light disperses largely to a controlled variable applied externally.
In a case of a scan type exposure apparatus having a light source of an excimer laser, for example, since the number of light pulses necessary for exposure of one shot is about fifty (50), if the emitted light intensity varies by one pulse due to dispersion of the intensity of each pulse, then a quantized error results and, by plural light pulses, a deterministic error of an integrated exposure amount is produced.
In an exposure apparatus which uses a light source comprising a pulse light source such as an excimer laser wherein the emitted light intensity varies with emissions of light, as compared with an exposure apparatus having a conventional light source of a Hg lamp, for example, it is not easy to make uniform the integrated exposure amount upon a substrate to be exposed.
In scanning exposure apparatuses, the integrated exposure amount in an arbitrary unit scan exposure region involves a non-correctable residual error which is provided by an intensity error of a last pulse light emitted last in the process of scan exposure. This causes non-uniformness of exposure. If the emitted light intensity of each pulse is lowered and, on the other hand, the light emission frequency of the pulse light source is increased, the error by the last emitted pulse light becomes relatively small, relative to the integrated exposure amount. Thus, the exposure non-uniformness may be reduced.
However, there is an upper limit to the light emission rate of a pulse light source. Currently, about 400 Hz is the upper limit. As a result, if the method described above is to be used to make the integrated exposure amount uniform, the emitted light intensity of the pulse light source as well as the scan speed have to be lowered to enlarge the number of average light pulses received by a unit scan exposure region. This necessarily results in slower throughput.
On the other hand, in the manufacture of a semiconductor device, exposure processes of a number ten (10) to twenty (20) are repeatedly executed to one semiconductor substrate (wafer). These exposure processes have different linewidth precisions and alignment precisions to patterns to be printed by respective processes. In consideration of this, a method has been proposed in which different exposure apparatuses are used for a critical layer where a high precision is required and for a rough layer where a precision as high as that for the critical layer is not required. As for the exposure apparatus for the exposure process of rough layers, an apparatus which enables a high throughput, though precision is not required therefor, has been used.
It is an object of the present invention to provide an exposure apparatus which enables optimization of performance such as throughput or exposure precision, for example, as required to an exposure apparatus, in accordance with the fineness of a pattern, this being able to be done by changing a parameter related to scan exposure on the basis of a tolerance ratio of exposure non-uniformness.
In accordance with an aspect of the present invention, there is provided an exposure apparatus, comprising: a light source for providing pulse light; mask scanning means for scanning a mask having a pattern; wafer scanning means for scanning a wafer onto which the pattern is to be projected; wherein said mask scanning means and said wafer scanning means serve to scan the mask and the wafer in a timed relation so that the mask is illuminated while superposing portions of an illumination region defined by the pulse light and being narrower than the pattern such that the pattern is lithographically transferred onto the wafer; and at least one of (i) scan speed determining means for determining scan speed of the mask and the wafer, (ii) light emission period determining means for determining the period of emission of the pulse light, (iii) illumination range determining means for determining the range of illumination for the mask, and (iv) position determining means for determining the position of illumination range limiting means with respect to a direction of an optical axis, such that a parameter related to scan exposure such as the scan speed, the light emission period, the illumination range or the position of the illumination range limiting means with respect to the optical axis direction, can be made variable on the basis of a tolerance ratio of exposure non-uniformness.
The tolerance ratio of exposure non-uniformness may preferably be determined on the basis of a minimum linewidth of the pattern.
The tolerance ratio of exposure non-uniformness may preferably be larger with a wider minimum linewidth of the pattern.
In an exposure apparatus according to the present invention, if there is a margin to the tolerance ratio of exposure non-uniformness, the light emission period of the light source may be prolonged while maintaining the exposure non-uniformness at about a tolerable level, by which the lifetime of the light source can be prolonged.
With an exposure apparatus according to the present invention, accurate manufacture of a device such as a semiconductor device, a liquid crystal device, an image pickup device or a magnetic head, for example, is enabled.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.